The present invention relates, in general, to a rotary beam interrupter and scanner system for use with a high energy light source, and more particularly to a method and apparatus for chopping a continuous laser beam into a plurality of bursts, or impulses, and directing such impulses to corresponding target areas, the system being arranged to repetitively scan the selected target areas. In a particular application of the system, the beam impulses are used to perforate multiple rows of holes or slots in a web traveling at a high rate of speed.
The present interrupter and scanner system is in the form of a plurality of rotary shutters which alternately block a beam of light or allow it to pass through selected portions to direct the beam to selected target areas. Shutter systems for light beams, and in particular for lasers, are generally well known, and usually take the form of, or incorporate, beam splitter systems. Some of these systems have been utilized for scanning purposes and are described in prior art publications such as U.S. Pat. No. 3,910,675. Because lasers offer the advantage of optically controllable, concentrated power, it has been proposed to use them for perforating webs, and such use is illustrated in the laser recording apparatus disclosed in U.S. Pat. No. 3,256,524. In most of the presently known laser devices which utilize an interrupted beam to produce spaced pulses or bursts of energy, the beam is interrupted by internally pulsing the laser, thereby producing intermittent, discrete energy bursts. Since a laser can be internally pulsed at very high frequencies, the time delay between pulses is sufficiently short for most uses of the beam. However, it has been found that even at the upper limits of the pulse frequency for lasers the repetition rate is not sufficiently high for some applications. For example, where it is desired to produce closely spaced perforations in a traveling web, the speed of the web is limited by the internal pulse capabilities of the laser, and it has been found that this places a relatively low limit on the rate at which such webs can be produced. Where production capabilities of perforated webs must be increased to meet demand, therefore, the only alternative presently available is the addition of duplicate laser systems, requiring considerable capital expenditure not only for the laser system, but for the increased space and personnel requirements. A prime example of an industry where such a problem is faced is the tobacco industry.
Perforated paper has been utilized for many years in the manufacture of cigarettes. Initially, the paper was perforated by mechanical devices such as one or more needles mounted for motion toward and away from a web of paper, the needle moving down to penetrate the web to form a hole. Although such devices form uniform, evenly spaced holes the mechanical motions were cumbersome and slow. More recently, electrical spark perforators have been provided wherein electrical arcs are produced to form the required hole. However, such devices do not produce uniform holes, and also have proven to be too slow to meet the requirements of the newly developed generation of high speed cigarette makers presently being utilized in the industry.
In order to meet the need for increased production, attempts have been made to use lasers for perforating the paper webs, for a laser has the advantage of permitting high speed operation and the light beam can be controlled by relatively reliable optical systems. Unfortunately, it has been found that although available laser devices can be pulsed at high frequencies, no commercially available units have been found capable of operating at a sufficiently high rate to obtain the web speeds now required to meet the needs of cigarette making machines for perforated paper. A limiting factor in the ability of a pulsed laser to operate at the required speed is the fact that where the laser is pulsed, it is working to produce a light beam only 50 percent of the time; thus, the beam is only available for about one-half of a given period and during the off period it is not capable of perforating the web. In an attempt to overcome this limitation, laser systems have been devised which utilize beam splitters, wherein partial reflectors divide an incoming beam into a plurality of beams. However, each time the beam is split by a partial reflector, the energy level of each resulting beam is correspondingly reduced not only by the fact that the beam has been split, but by the losses which occur in the splitters. Such beam splitters have been found unsatisfactory in web perforators since it is important to have a uniform energy level from all of the beams produced by the system so that the perforations in the web will be uniform. Further, since the energy level of the beam is reduced each time it passes through a splitter, the resultant beams have a relatively low energy level which in some cases is not sufficient to reliably produce uniform perforations.